Hi - it usually helps if you give us some context for where your phrase is taken from.
At a literal level I think this is about a stage / area for performance of some sort that is made of wood that is rough, unplaned, and generally looks a bit scruffy.
It could also be used in a metaphorical sense, as a platform can be a political position. A "rough-planked political platform" would also have qualities of being "thrown together" rather than highly polished and refined.
It is, I would guess, a platform constructed from wooden planks that are not very smooth. As well as the wood being rough, it is likely that the construction is somewhat crude. It's something like WMPG's "tree house" that I can see from my window. That is a flat platform about four feet square (just over a metre square) that we made one sunny afternoon from bits of left-over timber hastily screwed together. She did the design and gave the orders. I did the labouring.